The purpose of our studies is to develop new methods of gene transfer into hematopoietic stem cells using the genetic disease canine leukocyte adhesion deficiency (CLAD) as a model. We are using the canine model to identify new vectors for gene transfer and conditioning regimens to enable sufficient numbers of gene modified hematopoietic stem cells to engraft and reverse the disease phenotype. The canine form of this disease is an optimal model for these studies since: 1) the defect involves a membrane receptor on the surface of leukocytes, flow cytometry allows fascile detection and analysis of the number of gene corrected cells; 2) low levels of gene-corrected cells result in reversal of the disease phenotype; and 3) studies in the canine model have been predictive of success in humans in the field of hematopoietic stem cell biology. The presence of a human counterpart to the canine disease allows the results from the animal model to be directly extrapolated to humans. The long-term objective of these studies is to develop strategies that will allow levels of expression of CD18 in hematopoietic cells of children with leukocyte adhesion deficiency (LAD) that are sufficient to reverse the clinical phenotype. We have utilized this model to test retroviral and foamy viral gene transfer into the bone marrrow cells of dogs. The foamy viral vector demonstrated greater efficacy and a more favorable integration profile than the conventional retroviral vectors. The foamy viral vector-treated dogs are being followed for the durability of the correction, and for any possible genotoxicity from the vector. To date, there has been no genotoxicity. These results represent the first successful use of a foamy virus (FV) vector to treat a genetic disease, and they suggest that foamy virus vectors will be effective in treating human hematopoietic diseases. We are currently testing third-generation foamy viral vectors with cellular promoters and lentiviral vectors with cellular rather than viral promoters in our canine model.